drm/vc4: Fix sending of page flip completion events in FKMS mode.

In the rewrite of vc4_crtc.c for fkms, I dropped the part of the
CRTC's atomic flush handler that moved the completion event from the
proposed atomic state change to the CRTC's current state.  That meant
that when full screen pageflipping happened (glxgears -fullscreen in
X, compton, por weston), the app would end up blocked firever waiting
to draw its next frame.

Signed-off-by: Eric Anholt <eric@anholt.net>